Ans: B
Consider the mean of the weights of the seven boys, we have
Consider the mean of the weights of the seven boys, we have
$\begin{array}{rcl}
\dfrac{70+55+53+56+64+54+x}{7} & = & 58 \\
352 + x & = & 406 \\
x & = & 54
\end{array}$
Then we rearrange the seven data in ascending order, we have
$\begin{array}{c}
53\text{ kg},~54\text{ kg},~54\text{ kg},~55\text{ kg}, ~56\text{ kg},~64\text{ kg},~70\text{ kg}
\end{array}$
Hence, the median of their weights is $55\text{ kg}$.
